Monolithic Fiber-Integrated Diffractive Beam Splitter for Compact Single-Core to Multi-Core Coupling

We present a monolithic, diffractive beam splitter fabricated directly on a single-mode fiber facet using two-photon polymerization-based direct laser writing. The device acts as a proof of concept for coupling between single-mode and multi-core fibers using beam shaping and free-space projection over 40 μm. Using a waveguide-based mode expansion up-taper and diffractive optical elements, the input beam is split into seven beams over a full angular spread of 72<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>, and the outputs are collimated over a total structure length of 355 μm and a footprint of 115 μm diameter. The concept enables miniaturization of single-mode to multi-core fiber interfacing, for communication and lab-on-fiber applications.